This invention has to do with the field of color reproduction in general and, in particular to the process of producing hardcopy output on a four-color device from a digital image. Examples of four-color devices are: offset lithography in which the "four-color process" employs printing inks as colorants applied to paper, in a halftone screen pattern, by a printing press; gravure printing, which also employs printing inks applied to paper, off-press proofing systems, which employ toners as colorants to simulate the effect of an offset press; and computer-driven printers, which use a variety of technologies and colorants, such as jettable inks, toners, and dyes, applied in various continuous-tone, halftone, or dithered patterns.
These processes use three chromatic colorants which are commonly chosen to be the subtractive primaries cyan, magenta, and yellow (abbreviated as C, M, and Y). In addition, they may use an achromatic, or black, colorant (abbreviated as K).
In order to print digital images on these devices, it is necessary to be able to convert colors among different device color spaces. For example, consider a person who wishes to scan in a picture, view the picture on a monitor, and then write out the picture on a four-color offset press. In order to do this in such a way that the original picture, monitor image, and printed copy look the same, it is necessary to convert between the color spaces defined by the RGB measured by the scanner, the RGB used by the monitor, and CMYK used by the press. In order to produce this conversion, color management systems have been developed by a number of companies. These color management systems use device profiles to describe the colorimetric properties of devices. Each profile contains transformations between the device color space (i.e., the space defined by the amounts of each colorant used by the device) and a profile connection space (PCS). Standard choices for the PCS are the CIELAB and CIEXYZ color spaces defined by the International Commission on Illumination (CIE). In addition, profiles for output devices contain a simulation transform which maps from the PCS to the PCS for describing how out of gamut colors are mapped into the gamut of the output device before being printed. Thus each profile contains the following transforms:
(a) an input transform which maps the device color space to the profile connection space; PA1 (b) an output transform which maps the profile connection space to the device space; PA1 (c) a simulation transform which maps the PCS to the PCS, and describes how colors which are outside the gamut of the device are to be printed; and PA1 (d) a gamut alarm transform which maps the PCS to a single number and indicates whether or not a given color is within the gamut of the device.
The format for these profiles has been standardized by the International Color Consortium (ICC). A number of companies have produced and sold profiles which conform to this standard, and have also produced and sold applications which create ICC conforming profiles from measurement data. Thus, each owner of color processing equipment has available an increasing number of profiles from an increasing number of different sources.
In the case of a four-color device, such as an offset press, the output transform is not determined solely by the calorimetric properties of the output process. These processes are undetermined or, in other words, most printable colors can be printed with a variety of different combinations of cyan, magenta, yellow, and black colorants (CMYK). In general, these options can be thought of as different balances between the amount of black colorant (K) and the amount of a neutral combination (CMY) of the primary colorants. Since the printed colors are (or can be) identical, the choice among these options is made, not on the basis of colorimetry, but according to considerations of process control and repeatability, limits of the printing technology, cost, aesthetic taste, and the like. Some of the common approaches to the usage of K versus CMY are referred to in the art as Under-Color Removal (UCR) and Gray-Component Replacement (GCR).
Under current methods of generating profiles for output devices, the choice of GCR is one component of the process for generating the profile. An operator writes out a set of patches; measures those patches with a colorimeter; and gives those measured values to a computer program. The operator also gives parameters specifying the desired degree of GCR to that program. The computer program then uses the measured data and the choice of GCR to compute the profile. If the operator wishes a different amount of GCR, it is necessary to re-run the program. This is not too inconvenient if the operator has access to the software and to the measurement data which had been used in creating the output profile. However, in many cases, a person might own a profile which was generated by somebody else. In those cases there isn't any current method for the owner of such a profile to change its level of GCR.
U.S. Pat. No. 5,359,436 by Dichter discloses a method of recomputing black for a picture which has had its CMY values readjusted. This patent, however, does not base its recomputation of black ink on a colorimetric model for a specific output device so that the recomputation is characteristic of that device. Rather it assumes a simple standardized model for how black is replaced by cyan, magenta, and yellow. Thus, it cannot, in general, guarantee that the color of a picture will be the same after the black ink has been recomputed.
Consequently, a need exists for improvements in the presently known and utilized method and apparatus for constructing a profile for a four-color output device.